Magnetic elements mounted on the corners of the screen of color television tubes for compensating for the effect of the terrestrial magnetic field

ABSTRACT

FOR COMPENSATING THE EFFECT OF THE TERRESTRIAL MAGNETIC FIELD IN COLOR TELEVISION TUBES, COILS OR OTHER MAGNETIC ELEMENTS, EXTENDING ALONG THE TWO SIDES OF THE CORNER OF THE TUBE SCREEN, ARE PLACED IN FRONT OF AND AT THE FOUR CORNERS OF THE LATTER. THEY PROVIDE A MAGNETIC FIELD PARALLEL TO THE TUBE AXIS AND WHOSE INTENSITY DECREASES FROM THE SCREEN CORNERS TOWARDS THE CENTER THEREOF.

United States Patent Inventor Jean Ciecierski Paris, France Appl. No 837,288

Filed June 27, 1969 Patented June 28, 1971 Assignee Thomson-CSF Priority July 16, 1968 France 159,318

MAGNETIC ELEMENTS MOUNTED ON THE CORNERS OF THE SCREEN OF COLOR TELEVISION TUBES FOR COMPENSATING FOR THE EFFECT OF THE TERRESTRIAL MAGNETIC FIELD 5 Claims, 9 Drawing Figs.

US. Cl 315/8, 317/157.5, 335/210 Int. Cl ..H0lf 13/00,

[50] Field of Search 315/8; 3l7/l57.5;335/21O [56] References Cited UNITED STATES PATENTS 2,925,524 2/1960 Heil 315/8 3,317,781 5/1967 Barbin et al. 315/8 3,340,443 9/1967 Rieth et a1. 3 l 5/8X 3,387,172 6/1968 Simon 315/8 Primary ExaminerRoy Lake Assistant Examiner-V. Lafranchi Attorney-Cushman, Darby and Cushman PATENTEUJUN28IBY: 3,588,583

SHEET 1 BF 2 J sL MAGNETIC ELEMENTS MOUNTED ON THE CORNERS OF THE SCREEN OF COLOR TELEVISION TUBES FOR COMPENSATING FOR THE EFFECT OF THE TERRESTRIAL MAGNETIC FIELD Color television tubes comprise, at one end of the sealed envelope thereof, a flat screen upon which there are deposited in a regularly alternating pattern contiguous vertical rectangular, green, blue and red strips, and at the other end three guns which respectively receive the signals corresponding to these colors. In certain tubes, there is provided between the guns and the screen a grid consisting of a flat sheet, parallel to the screen, of mutually parallel wires which are also parallel to the strips of the screen.

In operation, the electron beams issuing from each gun are accelerated towards the screen which is raised to a high positive potential in relation to the guns (some tens of thousands of volts for example). The impact area of the beam upon the screen is made as small as technologically possible.

As will be readily understood, it is essential that during the scanning of the screen by the three electron beams, the areas of impact of each of them should at all times remain entirely I within the strip of the corresponding color, failing which a signal corresponding to a given color, applied to a gun; would produce on the screen a spot of a different color.

One of the functions of the grid, which is raised to a potential of some thousands of volts positive in relation to the guns, is to concentrate the beams prior to their impact upon the screen, so as to minimize the area of this impact.

Due to various disturbing factors, despite the presence of this grid, this condition cannot always be achieved unless certain corrections are effected. This is the reason why, to the other essential elements of the tube, there is added a chromatic purity" electrode the potential of which in relation to the gun is determined so as to cancel out any deviation of the impact areas of the beams from the strips of the corresponding color, in particular where such deviations are liable to be particularly significant, that is to say at the edges of the tube.

However, even so there remains a permanent source of disturbance, which tends to deflect the areas of impact of the beams from the corresponding strips, namely the terrestrial magnetic field.

It is an object of this invention, to cancel out or at any rate to reduce in substantial measure, the effect of the terrestrial magnetic field upon color television tubes equipped with a focusing grid.

This effect is particularly marked at the four comers-of the screen.

According to the invention there is provided a device for compensating the effect of the terrestrial magnetic field in color television tubes, comprising four separate magnetic elements, respectively mounted at the four corners of the screen, for producing a magnetic field parallel to the tube axis and decreasing from the corners towards the screen center.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawing accompanying the ensuing description and in which:

FIG. 1 is a schematic view, in elevation and plan, of one embodiment of a color television tube of the kind to which the invention relates, with a portion of the tube cut away;

FIG. 2 diagrammatically shows the trajectory of an electron within the tube;

FIG. 3 is another illustration of the electron trajectories in a color television tube of a kind to which the invention applies;

FIG. 4 is another illustration of FIG. 3 showing the displacements of the points of beam impact on the screen.

FIGS. 5 and 6 show the effect of the terrestrial magnetic field upon the trajectories of electrons in a color television tube;

FIG. 7 is a schematic view of one embodiment of the device according to the invention;

FIG. 8 is a perspective view of one of the elements of the device ofFIG. 7; and

FIG. 9 is a diagram showing how the elements of the device of FIG. 7 are supplied.

The tube shown in FIG. 1 comprises a cone C,, a front wall F, carrying a screen E, a grid G formed by wires 3, an anode A and the chromatic purity or correcting electrode P.

FIG. 2 shows schematically, in the horizontal central plane of a tube, the trajectory t of an electron coming from a gun situated at O on the axis XX of the tube, the trajectory t making an angle ,8 with this axis; V, B and R are three contiguous phosphor strips, which are respectively green, blue and red.

The effect of the magnetic field upon the electron trajectories within a color television tube will be first considered.

The terrestrial magnetic field can be split into two ponents, one vertical and the other horizontal.

At a given location, the vertical component of the terrestrial magnetic field, has a constant intensity, and a constant orientation in relation to the different trajectories followed by the electron beam while the latter is sweeping the screen, and its effect on same, although varying from one trajectory to another, is constant in respect of each of them.

The situation is quite different in so far as the terrestrial magnetic field component located in the horizontal plane is concerned. The latter not only varies with the location as does the vertical component, but has an orientation, in relation to the trajectories of the electron beams, which varies with the orientation of the screen, that is to say the orientation of the television receiver. In other words, the disturbing effect of this component upon the electron trajectories varies with the position of the receiver, which position is a matter of random choice for the user and is therefore essentially variable. For two orientations at l with respect to one another, the disturbing effects due to this component are in opposite directions.

This component can in turn be split down into a component parallel to the tube axis and a component perpendicular thereto.

In the following, it will be assumed that the coil coaxial with the screen and disposed around same in the present day commercial tubes, in order to demagnetize the tube at the starting, ensures that in operation the former of said two horizontal components is compensated.

Accordingly, only the compensation of the two other components, namely the vertical component and the horizontal component perpendicular to the tube axis will be considered.

The effect of the vertical component is explained in F IG. 3.

This FIG., the plane of which is the central horizontal plane of the tube, illustrates in full line the central position 1 of the electron beam and the two extreme positions 2 and 3 of the same beam at the rightand left-hand edges of the screen, in the absence of any terrestrial magnetic field. The vertical component of this field, which is directed downwards the plan view of FIG. 1, has the effect of deflecting the beams of electrons which are moving at the velocity v, from the guns towards the screen, towards the left of the screen, as indicated by the trajectories marked in broken lines 1', 2', and 3'.

It is well known that for an electron of mass m and charge e moving at a velocity v perpendicular to the magnetic field H, the deflection takes place along a circular trajectory located in a plane perpendicular to the field and tangentially to the velocity, in the direction determined by by the Maxwells corkscrew law, with a radius given by Rqv/eI-I (1).

In order to determine the magnitude of the disturbances produced by the terrestrial magnetic field, some estimates have been made with a gun situated at O on the axis of the tube, in an equipotential gun-grid space.

The displacement of the impact area of the beam, due to the vertical component of the terrestrial magnetic field, has been determined by the Applicant, at a certain number of points of the screen, namely at points a and b and at the center C on the horizontal central line Hm, at the points 11,, b, and c, on the horizontal H, at the top of the screen, and at the points a b and c on the horizontal H at the bottom of the screen. The results are indicated in FIG. 5, where E represents the screen comas seen by the viewer and H r the vertical component of the terrestrial magnetic field. The arrows indicate the direction of displacement under the effect of the field H, and its amplitude measured in hundredths of a millimetre, each graduation on the arrows representing three-hundredths of a millimeter.

It will be observed, as FIG. 4 shows, that the displacements of the points of impact are more marked at the left-hand edge than at the right-hand edge of the screen, since although the radii of the trajectories are the same in both cases, as equation (1) indicates, all the quantities having the same values in both cases, they are differently orientated in relation to the screen. In FIG. 4, portions of the circles C (trajectory l of FIG. 3), C (2' FIG. 3), C (3' FIG. 3), have been marked, these being the circles along which the beams are deflected for B= and B being at a maximum at either side of the tube axis; in this FIG., d, d d

The effect, at these points, of the horizontal field component perpendicular to the tube, namely the component H, has likewise been evaluated; the results are plotted on the same scale in FIG. 6, by the full-line arrows for a field H directed from the right towards the left as seen by the viewer, and by the broken-line arrows in respect of an orientation 1 80 away from the latter.

The calculations, which are simple in the case of the horizontal central plane H,, of the tube as indicated in FIGS. and 6, involve only the angle B. For the points on the horizontals H and H it is necessary to consider a second angle 0 which is the angle between the trajectory of the beam and the horizontals of the vertical plane defined by the angle B. Correspondingly, the electron velocity in the equipotential gungrid space, must be split into its vertical component and its two horizontal components, the latter being respectively parallel and perpendicular to the axis of tube. It is easy to see which of these components are the ones upon which the two terrestrial magnetic field components in question, exercise an effect.

The object of the present invention is to achieve at least an approximative compensation of the effects illustrated in FIGS. 5 and 6.

These effects become more marked as one moves away from the center towards the periphery of the screen and reach their maxima at the corners of the screen, where angles [3 and 0 are at a maximum. It is for this reason that, in accordance with the invention, the compensation of these efiects is effected by four elements located at the four corners of the screen, that is to sayat the points where the compensation required is at a maximum. The effect of the device in accordance with the invention is thus substantially different from that exercised uniformly by the coil provided in known systems around the screen and referred to hereinbefore. With this novel device, the correcting effect is concentrated at the comers of the screen, that is to say at the point where, the correction to be effected is greatest, which correction decreases rapidly towards'the center of the screen.

In the example of FIG. 7, the device in accordance with the invention is formed by four flat coils, independent of one another and located at each comer of the screen, shaped to follow the screen profile. These coils are placed against the screen externally thereof and outside its useful area. The coils B,, B B B have their axes parallel to that of the tube and produce a field parallel thereto. One of them is illustrated in perspective in FIG. 8. This coil has the particular shape shown: it comprises two horns normal to each other so as to fit into the comer of the screen in which it exerts its action.

The field produced is illustrated by the arrow f directed towards the rear of the FIG. at the point A; it decreases progressively towards the center C of the screen. This field exerts upon the beams, an effect which is the opposite of the effect of the terrestrial magnetic field. For a beam the impact point of which is at A in FIG. 7 in the top right-hand comer of the screen, each electron has a velocity which can be split into three components, one vertical and the other two horizontal, the latter two being respectively directed along the axis and perpendicular to the axis of the tube.

The effect of the magnetic field, directed along f, of the coil 3,, the only one to be considered since the others produce no perceptible effect in the corner considered, upon the vertical component of the beam is to deviate the latter in the horizontal plane towards the right of the screen, while the effect on the horizontal component of the beam deviates the latter upwards. The effect of this coil at A is thus opposite to that of the terrestrial magnetic field.

This effect is slightly modified because of a velocity component along the tube axis, i.e. in the direction of the magnetic field of the coil 8,, the trajectories along which the beam is deflected being helical and not circular as it would be if only the vertical component would exist. However, this does not substantially alter the foregoing conclusion. AS far as the electron velocity component perpendicular to the tube axis, is concerned, it introduces into the deflection of the beam a component which is directed vertically, that is to say in the direction of the phosphor strips, and therefore has no effect upon the lateral deflection.

Similar reasoning applies to the other corners of the screen.

As far as the effect of the component H,, of the terrestrial magnetic field is concerned, the latter resulting in deviations in the same direction as those resulting from the component H,., as a comparison of FIGS. 5 and 6 shows, it is apparent from what has been said before that the fields produced by the correcting coils also enable a correction to be efiected. Actually, what has to be considered in relation to a given point of the screen, is the sum of the two disturbances which are due to the components H, and H, as set out in FIGS. 5 and 6.

It will be seen, for example, that the total compensation to be effected at the top right-hand corner of the screen (point a on FIGS. 5 and 6) is l2+3=15/l00 mm, where the component H is directed as shown in FIG. 6, and l23=9/l00 mm where it is directed in the opposite direction, that is to say where the tube is swung through from its former position. It will be understood therefore that the current in the coils B B B B.,, will have to be adjusted as a function of this orientation. The television set should therefore comprise to this end, where independent coils are used, a control knob for each coil, which the viewer must reset when he changes the position of his receiver.

The controls will also have to be reset each time the set is moved from one location to another, where the terrestrial magnetic field has different components. The values indicated in FIGS. 5 and 6 are those found by the Applicant at the spot where he performed the tests.

This latter adjustment can be effected in the workshop by means of controls which are not apparent, prior to the delivery of the set, but external knobs have to be provided nevertheless, in order to effect resetting as a consequence of a change in the position of the set at a given spot.

The improvement which, other things being equal, is secured in color television tubes by the device in accordance with the invention, can be evaluated by the acceptable tolerance on the potential of the electrode, responsible for correcting chromatic purity (FIG. I), mention of which has been made earlier.

Tubes equipped with the device in accordance with the invention, enable a wider tolerance in this respect. Thus, for a series of 19 tested tubes, 17 enabled a tolerance of better than 50 volts on the potential in question, when equipped with the correcting device of the invention, whilst when not so equipped only 4 of them permitted such a tolerance.

Hereinafter, some of the characteristics of the coils used for the 49 cm screen of a tube in which-L=325 mm and D=23 mm (FIG. 2), operating with applied voltages of +l0,000 volts, on the anode, 8.000 volts on the grid and 25,000 volts on the screen, are given.

COIL DIMENSIONS (FIG. 8):

I, 44 mm; number of turns 200;

I 55 mm current: varying from 0 to I00 mA; L mm in one direction or the other.

L 120 mm h 20 mm.

FIG. 9 provides an example of the way in which four independent coils B,, B B B are supplied from a source S (12 volts), through potentiometers p P p P ln the example described hereinbefore, the compensating eflect which it is an object of the invention to achieve is obtained by means of electromagnetic coils with a variable current. In a modification of the invention, these coils can be replaced by permanent magnets fitted with adjustable shunts; by varying the position of the shunts in the magnetic circuit, the compensating field can be varied.

Of course, the invention is not limited to the embodiments described and shown which were given only by way of example but it is defined by the claims set forth below.

I claim:

1. A device for compensating the effect of the terrestrial magnetic field in color television tubes, comprising a color television tube screen and four separate magnetic elements,

respectively mounted in front of and at the four corners of the screen of the television tube for producing a magnetic field parallel to the tube axis and decreasing from the corners towards the screen center.

2. A device as claimed in claim 1, in which said elements are coils the axes of which are substantially parallel to the tube axis, and the section of which extends in each case along two sides of the screen from the relevant corner, means being provided for adjusting the current flowing through said coils.

3. A device as claimed in claim 2, in which said coils are separately supplied and separately controlled.

4. A device as claimed in claim 2, in which said coils are connected in series.

5. A device as claimed in claim 2, wherein said coils comprise a central portion and two horns, perpendicular to each other and extending along the two sides of the screen corner in which the coil is located. 

